**6. CdTe-based Schottky diode X-ray detectors for medical imaging**

In the section the possibilities of using diode structures based on relatively low resistivity p-CdTe and n-CdTe (*ρ* = 103 –104 Ω·сm), and polycrystalline CdTe in direct-conversion digital flat-panel X-ray image detectors are discussed.

Investigation of electrical properties, charge collection processes and the spectral distribution of the detection efficiency of X/γ-ray detectors based on CdTe with relatively low resistivity and Schottky contact confirm that their characteristics are generally inferior to technologically more complicated detectors with diodes Schottky based on semi-insulating CdTe. In particular, the currents of Schottky diodes under study are rather small for this type of diode structures—about 1 nA with a contact area of 3.5 mm2 . The current are determined by the generation-recombination in the SCR according to the Sah-Noyce-Shockley theory [22, 26].

The total detection efficiency for a detector utilizing a Schottky diode is the sum of the drift and diffusion components [27, 28]. As shown in **Figure 7a**, the contribution of the diffusion component to the total efficiency of the detector is quite important at *τ*<sup>p</sup> = 10−6 s and in the case of high-energy photons (i.e., at lower absorption coefficients) it is dominant. The efficiency of charge collection of the X/γ-rays detector with the Schottky diode substantially depends on the lifetime of charge carriers *τ* and concentration of uncompensated donors *N*d - *N*<sup>a</sup> (**Figure 7b**, inset). If *N*d - *N*<sup>a</sup> value is ~1014 сm−3, in order to ensure a practically complete charge collection (>99%), the lifetime of the carriers should equal or exceed ~10−7 s. If *N*d - *N*<sup>a</sup> ≈ 1016 см−3, the lifetime of charge carriers should not exceed 10−9 s, which is quite real when using even poor quality CdTe crystals. For X-ray examination of a breast, in the photon energy region *hν* < 30 keV n-type CdTe detector is more acceptable (**Figure 7b**). However, for X-ray examination of a chest (*hν* > 50 keV) p-CdTe should be chosen (**Figure 7c**). The detection efficiency of X-ray in the Al/p-CdTe diode structure at the maximum possible electron lifetime (a few microseconds) is 50–70 and 20–40% in the photon energy ranges of 20–30 and 50–80 keV, respectively (**Figure 7c**). Such characteristics seem to be acceptable for mammography and chest radiography.

**7. Conclusions**

*N*d - *N*<sup>a</sup>

**1.** The unconventional peculiarities, which are important from scientific and practical points of view, have been revealed by the experimental studies of the temperature dependences of resistivity and Fermi level energy of semi-intrinsic Cd0,9Zn0,1Te:In and CdTe:Cl crystals used for fabrication of X/γ-ray detectors: (i) the material can be semi-insulating when the compensation degree of a deep impurity level located near the middle of the band gap is around 0.5 (in this case, the Fermi level is pinned); (ii) if the impurity level is not close to the middle of the band gap, the semi-insulating condition is reached at low or high compensation degree (in this case, the Fermi level position strongly depends on the temperature *T* and activation energy Δ*E* can be significantly higher than one-half of the band gap at *T* → 0 K as it takes place in an intrinsic semiconductor). Among other things, this can lead to inversion of the conductivity type of the semiconductor as the temperature varies during climatic operation of a device that leads to qualitative changes of the electric

**Figure 7.** (a) The drift *η*drift and diffusion *η*dif components of the detection efficiency in the Schottky diode and their sum *η*. (b) Comparison of the total detection efficiency of the n- and p-CdTe-based Schottky diodes with the carrier lifetime 10−9 s. The inset shows the correlation between the charge-carrier lifetime *τ* and concentration of uncompensated donors

The photon energy ranges which are used for diagnostics of breast and chest are shown by shading.

 at which the collection of 99 and 95% charge carriers photogenerated at the interface between the depleted region and neutral part of the diode structure (*x = W*) is achieved at *V* = −100 V. (c) The detection efficiency of the Ni/n-CdTe and Al/p-CdTe diodes calculated for the uncompensated impurity concentration of 1016 cm−3 and carrier lifetime *τ* = 3 × 10−6 s.

Mechanisms of Charge Transport and Photoelectric Conversion in CdTe-Based X- and Gamma-Ray Detectors

http://dx.doi.org/10.5772/intechopen.78504

43

**2.** The features of operation of X/γ-ray detectors based on Cd(Zn, Mn)Te crystals with two Ohmic contacts have been established and ways to improve their energy resolution have been determined by comprehensive investigation of the electrical characteristics and quantum efficiency of detectors: (i) in the CdTe detector a rapid rise of the current with increasing voltage higher than 6–8 V for the crystal thickness of 1 mm is caused by the SCLC. A distinctive feature of the current is its temperature independence because the mechanism of injection of charge carriers is tunneling through the thin insulating film between the crystal and metal contact; (ii) thermoelectric cooling, commonly used for CdTe Schottky diode detectors, does not provide the desired result since it leads to a decrease in leakage

properties of both Schottky and Ohmic contacts in X/γ-ray detectors.

The use of a stacked CdTe detector with a Schottky diode, which is already practiced, can significantly improve the detecting efficiency of the device especially in the high-energy range of the spectrum. In the energy range ~100 keV the efficiency of a stacked detector is greater than that of a single layer detector [28]. The highly developed technology of the deposition of polycrystalline CdTe layers of large area with a surface-barrier structure in solar cells can be adapted to the fabrication of flat-panel X-ray image detectors. The presence of a barrier structure in the relatively low resistivity CdTe (*ρ* = 104 –106 Ω·cm) provides a low-leakage (dark) currents comparable with those in a-Se photoconductors (*ρ* = 1012 Ω·cm at 300 K). In a CdTe diode structure, virtually full charge collection occurs independently of the applied voltage at the carrier lifetime *τ* > 10−7 s and uncompensated impurity concentration higher than 1014 cm−3. Electric field concentration in the space charge region of a barrier structure eliminates the problem of the collection of charge generated by X-ray photon absorption (there is no need to increase the operating voltage up to several kiloelectronvolts as in the case of a-Se photoconductors).

Mechanisms of Charge Transport and Photoelectric Conversion in CdTe-Based X- and Gamma-Ray Detectors http://dx.doi.org/10.5772/intechopen.78504 43

**Figure 7.** (a) The drift *η*drift and diffusion *η*dif components of the detection efficiency in the Schottky diode and their sum *η*. (b) Comparison of the total detection efficiency of the n- and p-CdTe-based Schottky diodes with the carrier lifetime 10−9 s. The inset shows the correlation between the charge-carrier lifetime *τ* and concentration of uncompensated donors *N*d - *N*<sup>a</sup> at which the collection of 99 and 95% charge carriers photogenerated at the interface between the depleted region and neutral part of the diode structure (*x = W*) is achieved at *V* = −100 V. (c) The detection efficiency of the Ni/n-CdTe and Al/p-CdTe diodes calculated for the uncompensated impurity concentration of 1016 cm−3 and carrier lifetime *τ* = 3 × 10−6 s. The photon energy ranges which are used for diagnostics of breast and chest are shown by shading.

#### **7. Conclusions**

**6. CdTe-based Schottky diode X-ray detectors for medical imaging**

p-CdTe and n-CdTe (*ρ* = 103

42 New Trends in Nuclear Science

(**Figure 7b**, inset). If *N*d - *N*<sup>a</sup>

chest radiography.

case of a-Se photoconductors).

flat-panel X-ray image detectors are discussed.

structures—about 1 nA with a contact area of 3.5 mm2

structure in the relatively low resistivity CdTe (*ρ* = 104

In the section the possibilities of using diode structures based on relatively low resistivity

Investigation of electrical properties, charge collection processes and the spectral distribution of the detection efficiency of X/γ-ray detectors based on CdTe with relatively low resistivity and Schottky contact confirm that their characteristics are generally inferior to technologically more complicated detectors with diodes Schottky based on semi-insulating CdTe. In particular, the currents of Schottky diodes under study are rather small for this type of diode

generation-recombination in the SCR according to the Sah-Noyce-Shockley theory [22, 26].

The total detection efficiency for a detector utilizing a Schottky diode is the sum of the drift and diffusion components [27, 28]. As shown in **Figure 7a**, the contribution of the diffusion component to the total efficiency of the detector is quite important at *τ*<sup>p</sup> = 10−6 s and in the case of high-energy photons (i.e., at lower absorption coefficients) it is dominant. The efficiency of charge collection of the X/γ-rays detector with the Schottky diode substantially depends on the lifetime of charge carriers *τ* and concentration of uncompensated donors *N*d - *N*<sup>a</sup>

collection (>99%), the lifetime of the carriers should equal or exceed ~10−7 s. If *N*d - *N*<sup>a</sup> ≈ 1016 см−3, the lifetime of charge carriers should not exceed 10−9 s, which is quite real when using even poor quality CdTe crystals. For X-ray examination of a breast, in the photon energy region *hν* < 30 keV n-type CdTe detector is more acceptable (**Figure 7b**). However, for X-ray examination of a chest (*hν* > 50 keV) p-CdTe should be chosen (**Figure 7c**). The detection efficiency of X-ray in the Al/p-CdTe diode structure at the maximum possible electron lifetime (a few microseconds) is 50–70 and 20–40% in the photon energy ranges of 20–30 and 50–80 keV, respectively (**Figure 7c**). Such characteristics seem to be acceptable for mammography and

The use of a stacked CdTe detector with a Schottky diode, which is already practiced, can significantly improve the detecting efficiency of the device especially in the high-energy range of the spectrum. In the energy range ~100 keV the efficiency of a stacked detector is greater than that of a single layer detector [28]. The highly developed technology of the deposition of polycrystalline CdTe layers of large area with a surface-barrier structure in solar cells can be adapted to the fabrication of flat-panel X-ray image detectors. The presence of a barrier

(dark) currents comparable with those in a-Se photoconductors (*ρ* = 1012 Ω·cm at 300 K). In a CdTe diode structure, virtually full charge collection occurs independently of the applied voltage at the carrier lifetime *τ* > 10−7 s and uncompensated impurity concentration higher than 1014 cm−3. Electric field concentration in the space charge region of a barrier structure eliminates the problem of the collection of charge generated by X-ray photon absorption (there is no need to increase the operating voltage up to several kiloelectronvolts as in the

–104 Ω·сm), and polycrystalline CdTe in direct-conversion digital

value is ~1014 сm−3, in order to ensure a practically complete charge

. The current are determined by the

–106 Ω·cm) provides a low-leakage


current no more than 2–3 times when the temperature lowers from 300 to 260–270 K, and further cooling loses its meaning; (iii) with thinning the semiconductor crystal, the ratio between the carrier drift length and crystal thickness increases that improves the efficiency of charge collection at relatively low bias voltage; (iv) in the CdZnTe and CdMnTe detectors under study, a rapid rise of the current with increasing voltage and temperature due to injection of charge carriers is observed, so these crystals are not suitable for fabrication of X/γ-ray detectors; (v) a significant increase in the energy resolution can be achieved by improving the quality of Cd(Zn, Mn)Te crystals and, as a result, increasing the charge carrier lifetime.

of 20–30 and 50–80 keV, respectively. Such characteristics seem to be acceptable for mam-

Mechanisms of Charge Transport and Photoelectric Conversion in CdTe-Based X- and Gamma-Ray Detectors

This research was supported by the Collaborative Project COCAE (Grant SEC-218000) of the European Community's Seventh Framework Programme and by the Collaborative Project SENERA (Grant SfP-984705) of the NATO Science for Peace and Security Programme. The authors express gratitude to all colleagues indicated as co-authors in Refs. [5-11, 15, 20, 21,

[1] Szeles C. CdZnTe and CdTe materials for X-ray and gamma ray radiation detector applications. Physica Status Solidi B. 2004;**241**(3):783-790. DOI: 10.1002/pssb.200304296

[2] Sordo SD, Abbene L, Caroli E, Mancini AM, Zappettini A, Ubertini P. Progress in the development of CdTe and CdZnTe semiconductor radiation detectors for astrophysical

[3] Triboulet R, Siffert P. CdTe and Related Compounds; Physics, Defects, Hetero-and Nano-Structures, Crystal Growth, Surfaces and Applications. Amsterdam, The Netherlands:

[4] Takahashi T, Mitani T, Kobayashi Y, Kouda M, Sato G, Watanabe S, Nakazawa K, Okada Y, Funaki M, Ohno R, Mori K. High-resolution Schottky CdTe diode detector. IEEE Transactions on Nuclear Science. 2002;**49**:1297-1303. DOI: 10.1109/TNS.2002.1039655

[5] Kosyachenko LA, Diequez E, Maslyanchuk OL, Melnychuk SV, Sklyarchuk OV, Sklyarchuk OF, Grushko EV, Aoki T, Lambropoulos CP. Special features of conductivity of semi-intrinsic CdTe and CdZnTe single crystals used in X- and γ-ray detectors.

Proceedings of SPIE. 2010;**7805**:78051-78059. DOI: 10.1117/12.862726

and medical applications. Sensors. 2009;**9**:3491-3526. DOI: 10.3390/s90503491

, Volodymyr Gnatyuk2

and Toru Aoki2

http://dx.doi.org/10.5772/intechopen.78504

45

mography and chest radiography.

23-28] for their contribution in carrying out the investigations.

\*, Stepan Melnychuk1

1 Yuriy Fedkovych Chernivtsi National University, Chernivtsi, Ukraine

2 Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan

\*Address all correspondence to: emaslyanchuk@yahoo.com

Elsevier; 2009, 550 p. ISBN: 9780080914589

**Acknowledgements**

**Author details**

Olena Maslyanchuk1

**References**


of 20–30 and 50–80 keV, respectively. Such characteristics seem to be acceptable for mammography and chest radiography.
